Nanorods are anisotropic nanocrystals having a long axis and a short axis. The nanorod's structural anisotropy gives rise to anisotropic physical characteristics, displaying polarized optical, electronic, magnetic, and electric properties at the nanometer scale. An ability to assemble these nanorods into ordered assemblies with sizes that range from the microscopic to the macroscopic scale is critical for development of devices and applications based on the anisotropic physical properties. The dimensions of the ordered nanorods assemblies limit the size of nanorod-based devices. The degree of order in a nanorods assembly further determines the overall quality of a device. These nanorods-based devices include solar cells, photodetectors, lasers, and polarized light emitting devices (LED). Specifically, polarized LEDs permit decreased energy requirements and improved contrast, particularly during daytime use, when used in display applications.
The construction of nanorod-based LEDs requires lateral alignment of colloidal nanorods on a larger area surface. To date, several techniques have been used to laterally align colloidal nanorods, including: Langmuir-Blodgett techniques; external electric field alignment; imposing interfacial tension between two immiscible liquid phases; stretching a nanorods filled polymer composite; and transferring nanorods assemblies from a water-air interface with a poly(dimethylsiloxane) (PDMS) stamp pad. However, these techniques have not resulted in large-sized nanorods assemblies, but only those with dimensions of only a few micrometers when there is a high degree of order. Presently, for example, the maximum polarization ratio for an LED constructed from a colloidal CdSe/CdS nanorods assembly is about 0.34, as disclosed in Rizzo, A. et al. “Polarized Light Emitting Diode by Long-Range Nanorod Self-Assembling on a Water Surface” Acs Nano 3, 1506-12, (2009). The polarization ratio of 0.34 is much smaller than that of a single CdSe/CdS nanorod (0.75), which suggests that CdSe/CdS nanorods were not well laterally aligned in the nanorods assembly of the LED.
Hence there remains a need to achieve a nanorods assembly that can be prepared with large dimensions and have good lateral alignment of the nanorods. Achievement of such well aligned nanorods assemblies could permit construction of large scale polarized LEDs and other devices with polarization ratios approaching or exceeding the polarization ratio of the individual nanorods that form the assembly.